Add gs_image_alpha_mode enum for requested alpha handling: straight,
premultiplied with SRGB conversion, and just premultiplied. Both
premultiplied settings behave the same if the image is not SRGB, linear
is assumed.
Add gs_image_file3_t to store the alpha mode.
Add srgb.h file with helper functions.
Clean up vec4.h to use helpers, and remove unused functionality.
Update FFmpeg image loader to perform premultiplication on load.
Code submissions have continually suffered from formatting
inconsistencies that constantly have to be addressed. Using
clang-format simplifies this by making code formatting more consistent,
and allows automation of the code formatting so that maintainers can
focus more on the code itself instead of code formatting.
